Part 14

The physical cause of the difference of color is still more or less a matter of mystery. Although we can not consider it proved that the red stars are cooling and “dying out” suns, as has been suggested, we may, I think, conclude that their temperature, although doubtless very high, must be lower than that of the white stars. We know that a bar of iron when heated to redness is not so hot as when raised to “white heat,” and although the analogy between hot iron and stellar photospheres may not be a perfect one, it seems probable that the higher the temperature of a star, the whiter its color will be. Most of the white stars, as Sirius, Vega, and those only yellow or slightly colored, show spectra of Secchi’s first and second types, while the great majority of the red stars exhibit banded spectra of the third and fourth types.

To this rule there are, however, like other rules, some notable exceptions. For instance, Aldebaran, Alpha Hydræ, Xi Cygni, and 31 Orionis, although distinctly reddish stars, show well-marked spectra of the second or solar type. On the other hand, Rho Ursæ Majoris and Omega Virginis, which, according to Dunér, are only slightly yellow, have well-marked spectra of the third type.

An apparent change of color seems in some cases to be well established. The supposed red color of Sirius in ancient times is well known. A certain established change is found in the case of the famous variable star Algol, which is distinctly described as red by Al-Sûfi in the Tenth Century. It is now pure white, or nearly so, and this is probably the best attested instance on record of change of color in a bright star.

Schmidt’s Nova Cygni of 1876 was noted as “golden yellow” on the night of its discovery. When it had faded to the eighth magnitude, Dr. Copeland called it “decided red,” but when examined at Lord Crawford’s observatory in September, 1877, its color was recorded as “faint blue”! The new star in the Andromeda nebula was considered to be yellowish or reddish by most observers when near its maximum, but about a month later its color was noted as “bluish.”

Among the red and variable stars, there are many suspected cases of color variation. Espin and other observers have noted that the wonderful variable Mira Ceti is much less red at maximum than at minimum. My own observations confirm this. When at its maximum brightness, Mira does not seem to me a very highly-colored star, while at one of its minima I noted it as “fiery red.” Possibly, however, the great difference between its maximum and minimum brilliancy may have an influence on estimations of its color. The remarkable variable Chi Cygni is said to be “strikingly variable in color.” Espin’s observations in different years show it “sometimes quite red, at others only pale orange red.” The star Birmingham 118 was described by Schjellerup in 1863 as “decided red,” but it was found yellow by Secchi in 1868; “bluish” by Birmingham, 1873-76; “no longer red” by Schjellerup in March, 1876; and “white” by Franks in 1885. Espin omits it from his revised edition of Birmingham’s Catalogue.

Birmingham 169 was found red by Struve, blue or bluish-white by Birmingham in 1874, and white at Greenwich in the same year. Espin also saw it white in March, 1888. The star Birmingham 30, which lies close to Phi Persei (54 Andromedæ), was described by Schweizer as a “red star with a little disk” in January, 1843; Birmingham noted it as “light red” in December, 1875; Copeland “deep red” in January, 1876; and Dreyer “reddish” in September, 1878; but Espin, in November and December, 1887, found it “certainly not red, and nothing peculiar in the star’s appearance.” It might be expected that these curious changes of color, if real, would be accompanied by corresponding changes in the star’s spectrum. Such may be the case, and observations in this direction would probably lead to some interesting results.

There seems to be some law governing the distribution of the colored stars. The white stars appear to be most numerous, as a rule, in those constellations where bright stars are most abundant, for instance, in Orion, Cassiopeia, and Lyra; yellow and orange stars in large and ill-defined constellations, such as Cetus, Pisces, Hydra, Virgo, etc. The very reddish stars are most numerous in or near the Milky Way, and one portion of the Galaxy—between Aquila, Lyra, and Cygnus—was termed by Birmingham “the red region in Cygnus.”

Many of the stars when examined with a good telescope are seen to be double, some triple, and a few quadruple, and even multiple. These when viewed with the naked eye, or even a powerful binocular, seem to be single, and show no sign of consisting of two components. These telescopic double stars should be carefully distinguished from those which appear very close together with the naked eye, and which in opera-glasses or telescopes of small power might be mistaken for wide double stars by the inexperienced observer. These latter stars, such as Mizar—the middle star in the tail of the Great Bear—and its small companion, Alcor, have been called “naked-eye doubles,” but they are not, properly speaking, double stars at all. Telescopic double stars are far closer, and even the widest of them could not possibly be seen double without optical aid, even by those who are gifted with the keenest vision. Of these so-called “naked-eye doubles,” we may mention Alpha Capricorni, which on a very clear night may be seen with the naked eye to consist of two stars. On a very fine night two stars may be seen in Iota Orionis, the most southern star in Orion’s Sword. The star Zeta Ceti has near it a fifth magnitude star, Chi, which may be easily seen with the unaided vision. The star Epsilon Lyræ (near Vega) is a severe test for naked-eye vision. Bessel, the famous German astronomer, is said to have seen it when thirteen years of age. Omicron Cygni (north of Alpha and Delta Cygni) forms another naked-eye double, and other objects of this class may be noticed by a sharp-eyed observer.

The star Mizar, already referred to, is itself a wide telescopic double, and it seems to have been the first double star discovered with the telescope (by Riccioli in 1650). It consists of two components, of which one is considerably brighter than the other. It will give an idea of the closeness of even a “wide” telescopic double when we say that the apparent distance between Mizar and Alcor is nearly forty times the distance which separates the close components of the bright star. From this it will be seen that even a powerful binocular field-glass would fail to show Mizar as anything but a single star. The components may, however, be well seen with a 3-inch telescope, or even with a good 2-inch. The colors of the two stars are pale green and white. Between Mizar and Alcor is a star of the eighth magnitude, and others fainter. Mizar was the first double star photographed by Bond.

The Pole Star has a small companion at a little greater distance than that which separates the components of Mizar, but owing to the faintness of this small star, the object is not so easy as Mizar.

The star Beta Cygni is composed of a large and small star, of which the colors are described as “golden yellow and smalt blue.” This is a very wide double, and may be seen with quite a small telescope. Another fine double star is that known to astronomers as Gamma Andromedæ. The magnitudes of the components are about the same as those of Mizar, but a little closer. Their colors are beautiful (“gold and blue”). This is one of the prettiest double stars in the heavens. It is really a triple star, the fainter of the pair being a very close double star; but this is beyond the reach of all but the largest telescopes. The star Gamma Delphini is another beautiful object, the components being a little more unequal in magnitude, but the distance between them about the same as in Gamma Andromedæ. I have noted the colors with a 3-inch telescope as “reddish yellow and grayish lilac.” Gamma Arietis, the faintest of the three well-known stars in the head of Aries, is another fine double star, a little closer than Gamma Delphini. This is an interesting object, from the fact that it was one of the first double stars discovered with the telescope—by Hooke, in 1664, when following the comet of that year.

Another beautiful double star is Eta Cassiopeiæ, the components being about equal in brightness to those of Gamma Delphini, but the distance less than one-half. The colors are, according to Webb, yellow and purple; but other observers have found the smaller star garnet or red. This is a very interesting object, the components revolving round each other, and forming what is called a binary star.

Another fine double star is Castor, which is composed of two nearly equal stars separated by a distance about half that between the components of Gamma Andromedæ. This is also a binary, or revolving double star, but the period is long. Gamma Virginis is another fine double star, with components at about the same distance as those of Castor, and the colors very similar. It is also a remarkable binary star.

Among double stars of which the components are closer than those mentioned above, but which are within the reach of a good 3-inch telescope—a common size with amateur observers—the following may be noticed: Alpha Herculis, colors, orange or emerald green; the light of this star is slightly variable. Gamma Leonis, another binary star with a long period; colors, pale yellow and purple. Epsilon Boötis, a lovely double star, the colors of which Secchi described as “most beautiful yellow, superb blue.”

For observers in the Southern Hemisphere, the following fine double stars may be seen with a 3-inch telescope: Alpha Centauri; this famous star, the nearest of all the fixed stars to the earth, is also a remarkable binary; its period, as recently computed by Dr. See, is eighty-one years. Theta Eridani is a splendid pair, but closer than Alpha Centauri. It is, however, an easy object with a 3-inch telescope, and with a telescope of this size I noted the colors in India as light yellow and dusky yellow. The star known as ƒ Eridani is a very similar double to Theta, but the components are fainter. I noted the colors in India as yellowish-white and very light green.

Of triple, quadruple, and multiple stars, there are several which may be well seen with a small telescope. Of these may be mentioned Iota Orionis, the lowest star in the Sword of Orion, which consists of a bright star accompanied by two small companions. In Theta Orionis, the middle star of the Sword, four stars may be seen forming a quadrilateral figure, known to observers as the “trapezium.” There are two fainter stars in this curious object, which lie in the midst of the Orion nebula, but a somewhat larger telescope is required to see them. Within the trapezium are two very faint stars, which are only visible in the largest telescopes. In Sigma Orionis—a star closely south of Zeta, the lowest star in Orion’s Belt—six stars may be seen with a 3-inch telescope.

Double and multiple stars may be either optical or real. Optical double stars are those in which the component stars are merely apparently close together, owing to their being seen in nearly the same direction in space. Two stars may _seem_ to be close together, while, in reality, one of them may be placed at an immense distance behind the other. Just as two lighthouses at sea may, on a dark night, appear close together when viewed from a certain point, whereas they may be really miles apart. In the case of double stars it is, of course, always difficult to determine whether the apparent closeness of the stars is real or merely optical. But when, from a long series of observations of their relative position, we find that one is apparently moving round the other, we know that the stars must be comparatively close, and linked together by some physical bond of union. These most interesting objects are known to astronomers as binary, or revolving double stars. The probable existence of such objects was predicted from abstract reasoning by Mitchell in the Eighteenth Century; but the discovery of their actual existence was made by Sir William Herschel, while engaged on an attempt to determine the distance of some of the double stars from the earth. Unlike the planetary orbits, which are nearly circular, at least those of the larger planets of the Solar System, it is found that the orbits of these double stars differ, in many cases, widely from the circular form, in some cases, indeed, approaching in shape more the orbit of a comet than a planet.

The binary stars are among the most interesting objects in the heavens. The number now known probably amounts to nearly one thousand. In most of them, however, the motion is very slow, and in only about seventy cases has the change of position, since their discovery, been sufficient to enable an orbit to be computed.

Savary, in 1830, was the first astronomer who attempted to compute the orbit of a binary star, namely, the star Xi Ursæ Majoris. This remarkable pair was discovered by Sir William Herschel in 1780, and as the period of revolution is about sixty-one years, a considerable portion of the ellipse had been described in 1830, when it was attacked by Savary.

The binary star with the shortest period known at present seems to be the fourth magnitude star Kappa Pegasi. It was discovered as a wide double star by Sir William Herschel in 1786, the companion star being of the ninth magnitude. In August, 1880, Mr. Burnham, the famous American double star observer, examining the star with the 18½-inch refractor of the Dearborn Observatory, found the brighter star to be a very close double, with a distance between the components of only a quarter of a second of arc. A few years’ observations showed that this pair were in rapid motion round each other (about eleven years).

Another binary star, with a period of about the same length, is Delta Equulei, which was discovered to be a close double by Otto Struve in 1851. Next in order of shortness of period comes the southern binary star Zeta Sagittarii, for which an orbit was first computed in the year 1886 by the present writer. The orbit of this star will, I think, require still further revision, but the period of about eighteen years is probably not far from the truth.

Another remarkably rapid binary star is 85 Pegasi. Next in order of rapidity of motion we have the southern binary star 9 Argûs.

The star 42 Comæ Berenices has a period of about 25¾ years, according to Otto Struve. The orbit is remarkable from the fact that its plane passes through, or nearly through, the earth, and is, therefore, projected into a straight line, the companion star oscillating backward and forward on each side of its primary.

The star Beta Delphini—the most southern of the four stars in the “Dolphin’s Rhomb”—is also a fast-moving binary, discovered by Burnham in 1873. Burnham thinks the period will prove to be about twenty-eight years. The spectrum of the light of Beta Delphini is similar to that of our sun, so that the two bodies should be comparable in intrinsic brilliancy.

Another remarkable binary star with a comparatively short period is Zeta Herculis. This pair have now performed three complete revolutions since their discovery in 1782 by Sir William Herschel. Several orbits have been computed, but Dr. See’s period of thirty-five years is probably the best. The companion is, however, rather faint, being only 6½ magnitude, while the primary star is of the third.

In the case of the binary star, Eta Coronæ Borealis, it was, some forty years ago, uncertain whether its period was forty-three or sixty-six years, but now that two complete revolutions have been performed since its discovery by Sir William Herschel in 1781, the question has been finally decided in favor of the shorter period.

The brilliant star Sirius is also an interesting binary star. The companion, which is relatively very faint—about tenth magnitude—was discovered by Alvan Clark in 1862. The existence of some such disturbing body was previously suspected by astronomers, owing to observed irregularities in the proper motion of Sirius. Several orbits, giving periods of about fifty years, have been computed. The great brilliancy of Sirius, the brightest star in the heavens, naturally suggests a sun of great size. Recent investigations do not favor this idea. Its spectrum is, however, of the first type, and the star is therefore not comparable with the sun in brilliancy. The above result would indicate that stars of the first, or Sirian type, are intrinsically brighter than our sun.

Sirius is about eleven magnitudes brighter than its faint companion. This makes the light of Sirius about 25,000 times the light of the small star. The two bodies must, therefore, be differently constituted, and, indeed, the companion must be nearly a dark body. If Sirius has any planets revolving round it—like those of our solar system—they must forever remain invisible in our largest telescopes. This remark, of course, applies to all the fixed stars, single and double. They may possibly have attendant families of planets, like our sun, but if so, the fact can never be ascertained by direct observation.

The star Zeta Cancri is a well-known triple star, the close pair revolving in a period of about sixty years. Nearly two revolutions have now been completed since its discovery by Sir William Herschel in 1781. All three stars probably form a connected system, but the motion of the third star round the binary pair is very slow and irregular.

[Illustration: Fig. 18.—System of the Double Sun Alpha Centauri]

Another interesting binary star is Xi Ursæ Majoris. As already stated, this was the first pair for which an orbit was computed. More than a complete revolution has now been performed since its discovery by Sir William Herschel in 1780. The period has, therefore, been well determined, and seems to be about sixty years.

The bright southern star, Alpha Centauri, the nearest of all the fixed stars to the earth, so far as is known at present, is also a remarkable binary star. It seems to have been first noticed as a double star by Richaud in 1690.

Assuming my value of the sun’s stellar magnitude (about 27), I find that the sun, if placed at the distance of Alpha Centauri, would appear of about the same brightness as the star does to us. As, according to Professor Pickering, the spectrum of Alpha Centauri is of the second or solar type, it would seem that in mass, brightness, and physical condition the star closely resembles our sun.

We next come to another very interesting binary star, known to astronomers as 70 Ophiuchi. It is a very fine double star, the magnitudes of the components being about four and six, and the colors yellow and orange. More than a complete revolution has now been described by the components since its discovery by Sir William Herschel in 1779. Placed at the distance indicated by Krüger’s parallax, I find that our sun would be reduced to a star of about magnitude 3½, which shows that the sun and star are of about equal brightness. The spectrum is of the solar type, according to Vogel.

A very famous binary star is that known to astronomers as Gamma Virginis. Its history is a very interesting one. It lies close to the celestial equator, about one degree to the south and about fifteen degrees to the northwest of the bright star Spica (Alpha of the same constellation), with which it forms the stem of a Y-shaped figure formed by the brightest stars of the constellation Virgo, or the Virgin, Gamma being at the junction of the two upper branches. The brightness of Gamma Virginis is a little greater than an average star of the third magnitude. Variation of light has, however, been suspected in one or both components. The Persian astronomer, Al-Sûfi, in his description of the heavens, written in the Tenth Century, rates it of the third magnitude, and describes it as “the third of the stars of _al-auvâ_, which is a mansion of the moon,” the first and second stars of this “mansion” being Beta and Eta Virginis, the fourth star Delta, and the fifth Epsilon, these five stars forming the two upper branches of the Y-shaped figure above referred to. Gamma was called _Zawiyah-al-auvâ_, “the corner of the barkers!” perhaps from its position in the figure, which formed the thirteenth Lunar Mansion of the old astrologers. It was also called _Porrima_ and _Postvarta_ in the old calendars. The fact that Gamma Virginis really consists of two stars very close together seems to have been discovered by the famous astronomer, Bradley, in 1718. The rapid decrease in the apparent distance from 1780-1834 indicated that the apparent orbit is very elongated, and that possibly the two stars might “close up” altogether, and appear as a single star even in telescopes of considerable power. This actually occurred in the year 1836, or, at least, the stars were then so close together that the most powerful telescopes of that day failed to show Gamma Virginis as anything but a single star. Of course, it would not have been beyond the reach of the giant telescopes of our day. From the year 1836 the pair began to open out again.

Another interesting binary star is Eta Cassiopeiæ. Periods ranging from 149 to 222½ years have been found by different computers. The most recent computation makes it about 196 years.

The bright star Gamma Leonis, situated in the well-known “Sickle in Leo,” is also a binary star, but only a small portion of the orbit has been described since its discovery by Sir William Herschel in 1782. Dr. Doberck finds a period of 407 years. It is remarkable for its very high “relative brightness.” This pair forms a fine object for a small telescope.

The star known as 12 Lyncis is a triple star, the components being 5, 6, and 7½ magnitude. The close pair forms a binary system, for which an orbit has been computed by the present writer, who finds a period of about 486 years. Sir John Herschel predicted in 1823 that the angular motion of the pair would “bring the three stars into a straight line in 57 years.” This prediction was fulfilled in 1887, when measures by Tarrant showed that the stars were then exactly in a straight line.

The bright star Castor is a famous double star, and has been known since the year 1718, when it was observed by Bradley and Pond. It was also observed by Maskelyne in 1759, and frequently by Sir William Herschel from 1799 to 1803. Numerous orbits have been computed, with periods ranging from 199 years by Mädler and 1,001 years by Doberck. I find that the mass of the system of Castor is only 1/19th of the sun’s mass, a result which would imply that the components are masses of glowing gas! Dr. Bélopolsky has found, with the spectroscope, that the brighter component is a close binary star with a dark companion, like Algol. The period of revolution is about three days, and the relative orbital velocity about 20¾ miles a second. Dr. Bélopolsky’s observations show that the system is receding from the earth at the rate of about 4½ miles per second.